Vinnie Mirchandani at deal architect pointed out a Ray Wang article on the resale of unused licenses. My thoughts are in the comments on Ray’s article. But generally speaking, regardless of what Ray suggests, you can’t do it in the US (or the rest of the Berne Convention countries) under most licenses which have express prohibitions against it (you can almost always contract away your rights).
And, even if you could, your organization probably doesn’t have tracking enough to make it possible – just remember that if you overuse your permitted license count, chances are there’s another provision in your license that allows the vendor to charge you (perhaps at non-discounted pricing) for the overage.
What I DO like about Ray’s suggestion is that idea that you should try to negotiate for a recapture of maintenance fees on unused licenses. If you can’t resell them, the least you can do is take an annual count and only pay maintenance on the ones you’re using. There is, of course, trouble with this thought, too – as there are some vendors that used to allow this (the last one I remember was Autodesk). But the trouble is that you can get into a situation where you only upgrade SOME of your licenses to the current version because not all of them are currently covered by maintenance and the upgrades provided under such program.